In a fuel vapor treatment apparatus, fuel vapor evaporated in a fuel tank is temporarily adsorbed by a canister. Negative pressure in an intake passage of an engine is introduced into the canister so that the fuel vapor is desorbed and purged into the intake passage. JP-6-101534A shows a fuel vapor treatment apparatus in which a fuel vapor concentration of an air-fuel mixture is detected to control a purge amount of fuel vapor. Density of the air fuel mixture is detected in a purge passage which is for introducing the air-fuel mixture into the intake passage, and density of air is detected in an atmosphere passage opened to atmosphere. The fuel vapor concentration is calculated based on a ratio between the density of the air-fuel mixture and the density of the air. An orifice is respectively provided in the purge passage and the atmosphere passage. The densities of the air-fuel mixture and the air are calculated based on a differential pressure between both ends of the orifice. Thus, the density ratio is affected by the tolerance of each orifice. Besides, the density of the air-fuel mixture is detected while the air-fuel mixture is purged into the intake passage. Thus, the density of air-fuel mixture cannot be detected in a situation that the purge is not performed after the engine is started, so that the fuel vapor is hardly purged by a large amount in a short period.
The inventors have studied the technology in which pressure in a measure-passage with an orifice is reduced by air pump to introduce the air and the air-fuel mixture in a different timing so that the differential pressure between both ends of the orifice or the amount of air passing through the orifice is measured. The density ratio between the air and the air-fuel mixture is calculated based on the above measured result. According to this technology, the density ration can be detected by operating the air pump before purging, and only one orifice is used to detect the density ratio so that the tolerance of the orifice hardly affect on the measured result. However, according to the inventors' study, in a case that an orifice 1000 of which inner diameter is constant along the center axis thereof as shown in FIG. 17, following problems will arise.
Generally, density ρ of gas flowing through an orifice and a differential pressure ΔP between both ends of orifice have a relationship expressed by the following equation (1) by use of an air flowrate Q at the orifice, a cross-section area A, and a flowrate coefficient α.ρ=2·(α·A/Q)2·ΔP  (1)
In a case that the air flowrate Q corresponds to the suction amount of the air pump, the air flowrate Q can be derived from a characteristic of pressure (P)-flowrate (Q) of the air pump. In order to calculate the ratio between air density ρAir and the air-fuel mixture density ρGas, it is necessary to obtain an air differential pressure ΔPAir, an air-fuel mixture differential pressure ΔPGas, and an air flowrate coefficient αAir, and an air-fuel mixture flowrate coefficient αGas. When the coefficient αAir and the coefficient αGas are equal to each other, the ratio between the density ρAir and the density ρGas can be precisely calculated based on the measured differential pressures ΔPAir and ΔPGas. However, in the case that the orifice 1000 having a constant inner diameter is used, the inventors have found out that the coefficient αAir and the coefficient αGas are different from each other. Since the coefficient αAir and the coefficient αGas are physical value depending on the density ρAir and the density ρGas, the coefficients ρAir and αGas cannot be measure beforehand in calculating the density ratio. Thus, it must be assumed that the coefficient αAir and the coefficient αGas are equal to each other in order to calculate the ratio between the density ρAir and the density ρGas, so that the accuracy of calculating the ratio between ρAir and ρGas may be deteriorated.
In a case that the air pump is controlled in such a manner that the differential pressure ΔPAir and the differential pressure ΔPGas become equal to each other, it is necessary to obtain the flowrate QAir of air, and the flowrate QGas of the air-fuel mixture, and the coefficients αAir, αGas at the orifice. If the coefficient αAir and the coefficient αGas were equal to each other, the ratio between the density ρAir and the density ρGas could be precisely calculated. However, as described above, the coefficient ρAir and the coefficient αGas are different from each other in a case that the orifice 1000 is used.